Juno Reveals New Insights Into Io-Related Decameter Radio Emissions

The Juno mission is providing stunning new information about Jupiter and its environment. A new magnetic field model (JRM09) with much improved accuracy near the planet provides the basis for a better understanding of Io-related decametric radio emissions (DAM) and implications for auroral processes. Here, we selected Io-related DAM events observed by the Juno Waves instrument to shed light into the beaming angle, the resonant electron energy, and radio source location by forward modeling. We use the JRM09 model to better constrain the location and observability of DAM and characterize the loss cone-driven electron cyclotron maser instability. We obtained good agreement between synthetic and observed arcs with calculated beaming angles ranging from 33 degrees to 85 degrees and resonant electron energies up to 23 times higher than previously proposed. In addition, through a quantitative analysis, we provide an explanation regarding the higher likelihood of observing groups of arcs of Io DAM originating in the northern hemisphere relative to those originating in the southern hemisphere. This is primarily a consequence of the asymmetry of the magnetic field geometry, observer location, and pitch angles of the electrons at the equator. Plain Language Summary The interaction between Jupiter's magnetic field and Io has been observed from Earth via radio waves detection since 1955. The radio waves are generated by energetic electrons traveling along Jupiter's magnetic field and are observed when the instrument, Io, and the Jovian magnetic field are under particular geometric configurations. The radio waves may appear in groups, and it has been observed that those originating in the northern hemisphere are more abundant than their southern hemisphere counterparts. However, no explanation for this observation was proposed until now. Thanks to Juno, the geometry of the magnetic field has been better constrained as waves and magnetic field data have been continuously collected within the Jovian environment since July 2016. In this study, we estimate where the radio waves generate and the energy of the electrons that generate these waves, which is up to 23 times higher than previously proposed. We ultimately demonstrate that the geometry of Jupiter's magnetic field is a primary controller for the higher observation likelihood of radio wave groups originating in the northern hemisphere relative to those originating in the southern hemisphere.